/* Includes */
#include <unistd.h>		/*Symbolic Constants */
#include <sys/types.h>		/*Primitive System Data Types */
#include <errno.h>		/*Errors */
#include <stdio.h>		/*Input/Output */
#include <stdlib.h>		/*General Utilities */
#include <pthread.h>		/*POSIX Threads */
#include <string.h>		/*String handling */
#include <semaphore.h>		/*Semaphore */

/* prototype for thread routine */
void handler ( void *ptr );

/* global vars */
/* semaphores are declared global so they can be accessed
   in main() and in thread routine,
   here, the semaphore is used as a mutex */
sem_t mutex;
int counter; /* shared variable */


int main()
{
	int i[2];
	pthread_t thread_a;
	pthread_t thread_b;
	i[0] = 0; /* argument to threads */
	i[1] = 1;
	sem_init(&mutex, 0, 1);
	/*binary semaphore */
	/* initialize mutex to 1 - Second param = 0 - semaphore is local */

	/* Note: you can check if thread has been successfully
	created by checking return value of
	pthread_create */
	pthread_create (&thread_a, NULL, (void *) &handler,(void *) &i[0]);
	pthread_create (&thread_b, NULL, (void *) &handler,(void *) &i[1]);
	pthread_join(thread_a, NULL);
	pthread_join(thread_b, NULL);
	sem_destroy(&mutex); /* destroy semaphore */
	/* exit */
	exit(0);
} /* main() */


void handler ( void *ptr )
{
	int x;
	x = *((int *) ptr);
	printf("Thread %d: Waiting to enter critical region...\n", x);
	sem_wait(&mutex);
	/* down semaphore */
	/* START CRITICAL REGION */
	printf("Thread %d: Now in critical region...\n", x);
	printf("Thread %d: Counter Value: %d\n", x, counter);
	printf("Thread %d: Incrementing Counter...\n", x);
	counter++;
	printf("Thread %d: New Counter Value: %d\n", x,
	counter);
	printf("Thread %d: Exiting critical region...\n", x);
	/* END CRITICAL REGION */
	sem_post(&mutex);
	/* up semaphore */
	pthread_exit(0); /* exit thread */
}

